Abstract
This paper reviews four aspects of crystal growth theory: the nature of the rate-controlling process, the mechanismcontrolling molecular attachment onto the growing crystal surface, the nature of the crystal-melt interface, and the stability of planar interfaces relative to cellular interfaces. The rate-controlling process may be diffusion in the melt, heat flow, or the reaction at the crystal-melt interface. Diffusion or heat-flow controlled growth generally leads to a cellular morphology. For most silicates, interface-controlled growth leads to a faceted morphology. If the rate-controlling process is the interface reaction, the mechanism at the interface may be either continuous, with molecular attachment occurring at all points on the crystal surface, or lateral, with attachment occurring only on steps of the surface. The mechanism actually occurring can be determined by the dependence upon undercooling of the growth rate corrected for the viscosity of the melt. The nature of the interface can be described in terms of the interface roughness, which may be considered to be the topographic relief on the surface. Materials with small latent heats of fusion, such as quartz, should have molecularly rough interfaces and grow with a nonfaceted morphology, while materials with large latent heats, such as most other silicates, should have smooth interfaces and grow with a faceted morphology. The stability of planar interfaces relative to cellular interfaces can be discussed in terms of diffusional, heat-flow, surface-energy, and kinetic effects. For a freely growing crystal, such as in a magma chamber, only the surface-energy and kinetic effects aid in stabilizing planar interfaces. An attempt has been made to illustrate each of the phenomena discussed with a silicate example, although in one case an organic example is necessary.
